1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer which uses an electrophotographic system or an electrostatic recording system.
2. Related Background Art
When an image forming apparatus is used, and if an operating environment around the apparatus is different from a storage environment of paper as a recording material, or even if the operating environment is the same as the storage environment of the paper, it can be presumed that the image forming apparatus is operated in a state where a moisture amount in the atmosphere of the operating environment is different from a moisture absorption amount of the paper.
Generally speaking, when plain paper (neutral paper) used for copying or the like stands alone in each environment from a state of room temperature, the time required for allowing a sheet of paper to sufficiently adapt itself to the environment is about one hour. Even if the paper loaded in a cassette and supplied to an image forming apparatus adapts itself to the environment for a sufficiently long time, moisture is absorbed in and dehydrated from a portion near by the surface in each environment.
In the situation above, especially in a low humid environment in which an absolute moisture amount in a surrounding environment of an image forming apparatus is small, if a continuous image output or an intermittent image output within a fixed time is successively performed exceeding the fixed time (a pseudo continuous image output), the difference (moisture amount of apparatus surrounding environment less than moisture amount of paper) of this moisture amount may cause trouble for the image forming process.
In case of an image forming apparatus of an electrophotographic method, although transferring means uses a transferring belt, a roller or the like as a transferring member to transfer a toner image formed on a photosensitive member to paper, this transferring member is frequently used in contact with the photosensitive member or with a fixed amount of small gap retained. The toner image on the photosensitive member is efficiently transferred to the paper by making the paper adhere to the photosensitive member in a certain amount of adhesion state with this contacting or non-contacting transferring member.
When a continuous image output including a pseudo continuous image output is performed, a latent image potential change on a photosensitive member that is considered as influences, such as exfoliation discharge at transfer, a transfer voltage and the difference in a moisture amount of paper and an image forming apparatus occurs, and an image density change occurs.
A phenomenon in which a latent image on a photosensitive member is changed due to such continuous image output is a transient change unlike a degraded change until the life of an image forming apparatus expires. Accordingly, since the phenomenon is recovered when a certain measure of time elapses, the corrective action is difficult.
Generally, as remedy means against a latent image potential change on a photosensitive member, detection of environmental data, such as temperature and humidity, detection of the potential on the photosensitive member or detection of the density on the photosensitive member, a transferring material, an intermediate transferring member or paper is performed, and the density correction of an output image is performed based on each result.
If a potential sensor and a density sensor is not mounted on an image forming apparatus, such a method is usually used that makes an estimate to the extent of a process life and performs density correction based on the detection data of environmental parameters, such as temperature and humidity, using a potential change table and a development contrast on a previously prepared photosensitive member.
Further, in case of an image forming apparatus having no potential sensor and density sensor, there is also a method for reading an image density (a gradation pattern) using external reading means, such as a scanner, concerning an output image, and making the image forming apparatus perform correction according to a gradation correction table using communication means, as a measure for a density change.
However, since the density change under continuous output is a transient decrease, it is difficult to take a sufficient measure as long as such an image forming apparatus that always detects the potential on a photosensitive member and performs feedback control is not used. Moreover, if the correction described above is made by the reading means in such a situation in which a transient potential change on the photosensitive member occurs, the correction runs into a situation in which density correction is further necessary (refer to Japanese Patent Application Laid-Open No. 10-28229).
Hence, a measure for a change of a latent image on a photosensitive member, especially a decrease of an exposure portion potential (light portion potential Vl) that is directly linked with a density change, caused by a continuous image output using paper in a low humid environment, is necessary.
FIG. 7 shows a transition of a Dmax density at the time when a continuous image output is performed in a low humid environment at a temperature of 23xc2x0 C. and a humidity of 5%. This environment at the temperature of 23xc2x0 C. and the humidity of 5% is a low humid environment in which a moisture amount is 0.89 g/1 kg (a unit indicates g of water/l kg of air), and the moisture amount is less than 1 g/l kg.
It is known from FIG. 7 that density decreases when a continuous image output is performed, and the density is recovered when the continuous image output stops for a certain period.
A latent image potential change in a passed sheet portion on a photosensitive member at this time is shown in FIG. 8. It is known from by FIG. 8 that density decreases due to a continuous image output, and the density is recovered when the continuous image output stops for a fixed period. It is characteristic that while the potential (Vd) of the non-exposure portion is almost fixed, a change of the potential (Vl) of the exposure portion is large.
FIG. 9 shows an enlarged drawing of the exposure portion potential of FIG. 8. It is well known from FIG. 9 that a decrease amount of the exposure portion potential VI also reaches 40 to 50 V at a maximum. It is known that the potential Vl decreases even in the exposure portion in which no image is formed on paper.
Since a change of the exposure portion potential on this level has a great effect on a development contrast potential, it causes a density change of about xcex94D=0.1 or more as shown in FIG. 7.
As causes of a decrease of the exposure portion potential as shown in FIG. 9, various influence factors are considered. For example, a transfer voltage, exfoliation discharge in a transfer unit, wear between a photosensitive member and paper, a sensitivity change of the photosensitive member due to a temperature change, a sensitivity change of the photosensitive member due to a moisture absorption amount of paper or the like.
Table 1 shows the result on which the difference in a change of the exposure portion potential Vl according to the presence of 200 continuation passed sheets and the presence (ON/OFF) of transfer is examined in a low humid environment at a moisture amount of 0.89 g/kg.
Further, a change amount from an initial value of the exposure portion potential VI at the time when a continuous output of 1,000 sheets was performed with transfer ON, and a pseudo continuous output for a total of 1,000 sheets, intermittently 500 times every two sheets, was performed are shown in FIG. 10. Besides, two kinds of lots A and B of photosensitive members were prepared. The two kinds of the lots A and B were tested for the continuous output and the one kind of the lot B was tested for the pseudo continuous output.
It is known from FIG. 10 that a potential decrease of about 50 V occurs in the exposure portion potential Vl for the continuous output of 1,000 sheets. It is also known that a decrease of the exposure portion potential of the same trend also occurs in the pseudo continuous output of 1,000 sheets of two sheets intermittence. That is, when such a continuous image output is performed including a pseudo continuous output of which the output is repeated at short time intervals, it is known that a decrease of the same exposure portion potential occurs.
Table 2 lists the result on which the recursiveness of potential according to the elapsed 5 time of the exposure portion potential Vl of which the potential decreased was confirmed in the environment at a moisture amount of 0.89 g/kg.
As a result, it is known that when a decrease amount of the exposure portion potential is about 50 V at a maximum, the longest time is required until the exposure portion potential is completely recovered, but it is recovered about in ten minutes to the level where any influence can hardly be found. When the decrease amount is 50 V or less, the exposure portion potential is completely recovered in 10 minutes or less.
FIG. 11 shows the result on which the environmental dependence of a decrease amount of the Vl potential was examined. As shown in FIG. 11, as a low humid environment where a moisture amount of the environment is little is reached, a decrease amount of the exposure portion potential increases. As the environment exceeds a moisture amount of 7.6 g/kg, the decrease amount of the potential is lost.
Table 3 lists the result on which a decrease amount of the exposure portion potential Vl was examined at the time when an exposed amount is fixed against 700 V, 500 V and 300 V of the non-exposure portion potential Vd and 200 continuation sheets were passed in A4R (A4 size paper is conveyed lengthwise).
It was known from the result of Table 3 that as a latent image contrast potential becomes high, a decrease amount of the Vl potential increases. The paper used is an SK 65 g sheet.
Table 4 lists the result on which a decrease amount of the exposure portion potential Vl was examined by changing the kind of the recording material to which paper passes.
It was known that only when paper is used as a recording material, the exposure portion potential Vl decreases, and a decrease amount of the potential differs quite little even if the thickness and kind of the paper are changed. Further, it was confirmed that the potential will not decrease even in the condition free of a passed sheet of the recording material.
It was known from the result described above that a decrease amount of the exposure portion potential is largest when paper is passed as a recording material and transfer is in the on state.
Further, it was known that a decrease amount of the exposure portion potential changes according to a moisture amount of the apparatus surrounding environment, and not only in the state of a continuation passed sheet but also in the state of an intermittence passed sheet that can be regarded as the continuation passed sheet, that is, in the state of a pseudo continuation passed sheet, the potential decreases in accordance with the number of passed sheets and the decrease amount tends to be saturated in about 1,000 passed sheets.
Concerning causes of a decrease of the exposure portion potential, as described previously, a plurality of diversified factors are assumed to be affected. However, the occurrence is limited to the case where paper was used as a recording material in a low humid environment, and it is clear that the trend in the decrease of the potential is also proportional to the number of passed sheets.
An object of the present invention is to provide an image forming apparatus that can obtain a fixed density in a low humid environment.
Another object of the present invention is to provide an image forming apparatus that prevents a density change at the time when a continuous output is performed using paper as a recording medium.
A further object of the present invention is to provide an image forming apparatus, comprising:
an image bearing member for bearing an electrostatic image;
developing means for developing the electrostatic image on said image bearing member;
temperature and humidity detecting means for detecting temperature and humidity;
deciding means for deciding an image forming condition based on the detection output of said temperature and humidity detection means; and
correcting means for correcting the decision of the image forming condition by said deciding means in a low humid environment and at continuous image formation.
An even further object of the present invention will become clear in the following description.